Network Working Group T. Bradley
Request for Comments: 1294 C. Brown
Wellfleet Communications, Inc.
A. Malis
BBN Communications
January 1992
Multiprotocol Interconnect over Frame Relay1. Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
2. Abstract
This memo describes an encapsulation method for carrying network
interconnect traffic over a Frame Relay backbone. It covers aspects
of both Bridging and Routing. Systems with the ability to transfer
both this encapsulation method, and others must have a priori
knowledge of which virtual circuits will carry which encapsulation
method and this encapsulation must only be used over virtual circuits
that have been explicitly configured for its use.
3. Acknowledgements
Comments and contributions from many sources, especially those from
Ray Samora of Proteon, Ken Rehbehn of Netrix Corporation, Fred Baker
and Charles Carvalho of Advanced Computer Communications and Mostafa
Sherif of AT&T have been incorporated into this document. Special
thanks to Dory Leifer of University of Michigan for his contributions
to the resolution of fragmentation issues. This document could not
have been completed without the expertise of the IP over Large Public
Data Networks working group of the IETF.
4. Conventions
The following language conventions are used in the items of
specification in this document:
o Must, Shall or Mandatory -- the item is an absolute
requirement of the specification.
o Should or Recommended -- the item should generally be
followed for all but exceptional circumstances.
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RFC 1294 Multiprotocol over Frame Relay January 1992
o May or Optional -- the item is truly optional and may be
followed or ignored according to the needs of the
implementor.
5. Introduction
The following discussion applies to those devices which serve as end
stations (DTEs) on a public or private Frame Relay network (for
example, provided by a common carrier or PTT). It will not discuss
the behavior of those stations that are considered a part of the
Frame Relay network (DCEs) other than to explain situations in which
the DTE must react.
The Frame Relay network provides a number of virtual circuits that
form the basis for connections between stations attached to the same
Frame Relay network. The resulting set of interconnected devices
forms a private Frame Relay group which may be either fully
interconnected with a complete "mesh" of virtual circuits, or only
partially interconnected. In either case, each virtual circuit is
uniquely identified at each Frame Relay interface by a Data Link
Connection Identifier (DLCI). In most circumstances DLCIs have
strictly local significance at each Frame Relay interface.
The specifications in this document are intended to apply to both
switched and permanent virtual circuits.
6. Frame Format
All protocols must encapsulate their packets within a Q.922 Annex A
frame [1,2]. Additionally, frames shall contain information
necessary to identify the protocol carried within the Protocol Data
Unit (PDU), thus allowing the receiver to properly process the
incoming packet. The format shall be as follows:
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RFC 1294 Multiprotocol over Frame Relay January 1992
+-----------------------------+
| flag (7E hexadecimal) |
+-----------------------------+
| Q.922 Address* |
+-- --+
| |
+-----------------------------+
| Control (UI = 0x03) |
+-----------------------------+
| Optional Pad(s) (0x00) |
+-----------------------------+
| NLPID |
+-----------------------------+
| . |
| . |
| . |
| Data |
| . |
| . |
+-----------------------------+
| Frame Check Sequence |
+-- . --+
| (two octets) |
+-----------------------------+
| flag (7E hexadecimal) |
+-----------------------------+
* Q.922 addresses, as presently defined, are two octets and
contain a 10-bit DLCI. In some networks Q.922 addresses may
optionally be increased to three or four octets.
The control field is the Q.922 control field. The UI (0x03) value is
used unless it is negotiated otherwise. The use of XID (0xAF or
0xBF) is permitted and is discussed later.
The pad field is an optional field used to align the remainder of the
frame to a convenient boundary for the sender. There may be zero or
more pad octets within the pad field and all must have a value of
zero.
The Network Level Protocol ID (NLPID) field is administered by ISO
and CCITT. It contains values for many different protocols including
IP, CLNP and IEEE Subnetwork Access Protocol (SNAP)[10]. This field
tells the receiver what encapsulation or what protocol follows.
Values for this field are defined in ISO/IEC TR 9577 [3]. A NLPID
value of 0x00 is defined within ISO/IEC TR 9577 as the Null Network
Layer or Inactive Set. Since it cannot be distinguished from a pad
field, and because it has no significance within the context of this
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RFC 1294 Multiprotocol over Frame Relay January 1992
encapsulation scheme, a NLPID value of 0x00 is invalid under the
Frame Relay encapsulation. The known NLPID values are listed in the
Appendix.
For full interoperability with older Frame Relay encapsulation
formats, a station may implement section 15, Backward Compatibility.
There is no commonly implemented maximum frame size for Frame Relay.
A network must, however, support at least a 262 octet maximum.
Generally, the maximum will be greater than or equal to 1600 octets,
but each Frame Relay provider will specify an appropriate value for
its network. A Frame Relay DTE, therefore, must allow the maximum
acceptable frame size to be configurable.
The minimum frame size allowed for Frame Relay is five octets between
the opening and closing flags.
7. Interconnect Issues
There are two basic types of data packets that travel within the
Frame Relay network, routed packets and bridged packets. These
packets have distinct formats and therefore, must contain an
indication that the destination may use to correctly interpret the
contents of the frame. This indication is embedded within the NLPID
and SNAP header information.
For those protocols that do not have a NLPID already assigned, it is
necessary to provide a mechanism to allow easy protocol
identification. There is a NLPID value defined indicating the
presence of a SNAP header.
A SNAP header is of the form
+-------------------------------+
| Organizationally Unique |
+-- +---------------+
| Identifier | Protocol |
+---------------+---------------+
| Identifier |
+---------------+
All stations must be able to accept and properly interpret both the
NLPID encapsulation and the SNAP header encapsulation for a routed
packet.
The three-octet Organizationally Unique Identifier (OUI) identifies
an organization which administers the meaning of the Protocol
Identifier (PID) which follows. Together they identify a distinct
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RFC 1294 Multiprotocol over Frame Relay January 1992
Format of Bridged 802.6 Frame
+-------------------------------+
| Q.922 Address |
| Control 0x03 | pad(s) 0x00 |
+-------------------------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-05 or 0x00-0B |
+-------------------------------+
| Reserved | BEtag | Common
+---------------+---------------+ PDU
| BAsize | Header
+-------------------------------+
| MAC destination address |
+-------------------------------+
| (remainder of MAC frame) |
+-------------------------------+
| |
+- Common PDU Trailer -+
| |
+-------------------------------+
| FCS |
+-------------------------------+
The Common Protocol Data Unit (PDU) Header and Trailer are
conveyed to allow pipelining at the egress bridge to an 802.6
subnetwork. Specifically, the Common PDU Header contains the
BAsize field, which contains the length of the PDU. If this field
is not available to the egress 802.6 bridge, then that bridge
cannot begin to transmit the segmented PDU until it has received
the entire PDU, calculated the length, and inserted the length
into the BAsize field. If the field is available, the egress
802.6 bridge can extract the length from the BAsize field of the
Common PDU Header, insert it into the corresponding field of the
first segment, and immediately transmit the segment onto the 802.6
subnetwork. Thus, the bridge can begin transmitting the 802.6 PDU
before it has received the complete PDU.
One should note that the Common PDU Header and Trailer of the
encapsulated frame should not be simply copied to the outgoing
802.6 subnetwork because the encapsulated BEtag value may conflict
with the previous BEtag value transmitted by that bridge.
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RFC 1294 Multiprotocol over Frame Relay January 1992
If a station supporting XID receives an XID frame, it shall respond
with an XID response. In processing an XID, if the remote maximum
frame size is smaller than the local maximum, the local system shall
reduce the maximum size it uses over this DLC to the remotely
specified value. Note that this shall be done before generating a
response XID.
The following diagram describes the use of XID to specify non-use of
acknowledged mode multiple frame operation.
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RFC 1294 Multiprotocol over Frame Relay January 19929. Fragmentation Issues
Fragmentation allows the exchange of packets that are greater than
the maximum frame size supported by the underlying network. In the
case of Frame Relay, the network may support a maximum frame size as
small as 262 octets. Because of this small maximum size, it is
advantageous to support fragmentation and reassembly.
Unlike IP fragmentation procedures, the scope of Frame Relay
fragmentation procedure is limited to the boundary (or DTEs) of the
Frame Relay network.
The general format of fragmented packets is the same as any other
encapsulated protocol. The most significant difference being that
the fragmented packet will contain the encapsulation header. That
is, a packet is first encapsulated (with the exception of the address
and control fields) as defined above. Large packets are then broken
up into frames appropriate for the given Frame Relay network and are
encapsulated using the Frame Relay fragmentation format. In this
way, a station receiving fragments may reassemble them and then put
the reassembled packet through the same processing path as a packet
that had not been fragmented.
Within Frame Relay fragments are encapsulated using the SNAP format
with an OUI of 0x00-80-C2 and a PID of 0x00-0D. Individual fragments
will, therefore, have the following format:
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RFC 1294 Multiprotocol over Frame Relay January 1992
+---------------+---------------+
| Q.922 Address |
+---------------+---------------+
| Control 0x03 | pad 0x00 |
+---------------+---------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+---------------+
| OUI 0x80-C2 |
+---------------+---------------+
| PID 0x00-0D |
+---------------+---------------+
| sequence number |
+---------------+---------------+
|F| RSVD |offset |
+---------------+---------------+
| fragment data |
| . |
| . |
| . |
+---------------+---------------+
| FCS |
+---------------+---------------+
The sequence field is a two octet identifier that is incremented
every time a new complete message is fragmented. It allows detection
of lost frames and is set to a random value at initialization.
The reserved field is 4 bits long and is not currently defined. It
must be set to 0.
The final bit is a one bit field set to 1 on the last fragment and
set to 0 for all other fragments.
The offset field is an 11 bit value representing the logical offset
of this fragment in bytes divided by 32. The first fragment must have
an offset of zero.
The following figure shows how a large IP datagram is fragmented over
Frame Relay. In this example, the complete datagram is fragmented
into two Frame Relay frames.
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RFC 1294 Multiprotocol over Frame Relay January 1992
interrupted with other packets or information intended for the same
DLC. An end station must be able to re-assemble up to 2K octets and
is suggested to support up to 8K octet re-assembly. If at any time
during this re-assembly process, a fragment is corrupted or a
fragment is missing, the entire message is dropped. The upper layer
protocol is responsible for any retransmission in this case.
This fragmentation algorithm is not intended to reliably handle all
possible failure conditions. As with IP fragmentation, there is a
small possibility of reassembly error and delivery of an erroneous
packet. Inclusion of a higher layer checksum greatly reduces this
risk.
10. Address Resolution
There are situations in which a Frame Relay station may wish to
dynamically resolve a protocol address. Address resolution may be
accomplished using the standard Address Resolution Protocol (ARP) [6]
encapsulated within a SNAP encoded Frame Relay packet as follows:
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | pad(s) 0x00 |
+-----------------------+-----------------------+
| NLPID = 0x80 | | SNAP Header
+-----------------------+ OUI = 0x00-00-00 + Indicating
| | ARP
+-----------------------+-----------------------+
| PID = 0x0806 |
+-----------------------+-----------------------+
| ARP packet |
| . |
| . |
| . |
+-----------------------+-----------------------+
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RFC 1294 Multiprotocol over Frame Relay January 1992
Where the ARP packet has the following format and values:
Data:
ar$hrd 16 bits Hardware type
ar$pro 16 bits Protocol type
ar$hln 8 bits Octet length of hardware address (n)
ar$pln 8 bits Octet length of protocol address (m)
ar$op 16 bits Operation code (request or reply)
ar$sha noctets source hardware address
ar$spa moctets source protocol address
ar$tha noctets target hardware address
ar$tpa moctets target protocol address
ar$hrd - assigned to Frame Relay is 15 decimal
(0x000F) [7].
ar$pro - see assigned numbers for protocol ID number for
the protocol using ARP. (IP is 0x0800).
ar$hln - length in bytes of the address field (2, 3, or 4)
ar$pln - protocol address length is dependent on the
protocol (ar$pro) (for IP ar$pln is 4).
ar$op - 1 for request and 2 for reply.
ar$sha - Q.922 source hardware address, with C/R, FECN,
BECN, and DE set to zero.
ar$tha - Q.922 target hardware address, with C/R, FECN,
BECN, and DE set to zero.
Because DLCIs within most Frame Relay networks have only local
significance, an end station will not have a specific DLCI assigned
to itself. Therefore, such a station does not have an address to put
into the ARP request or reply. Fortunately, the Frame Relay network
does provide a method for obtaining the correct DLCIs. The solution
proposed for the locally addressed Frame Relay network below will
work equally well for a network where DLCIs have global significance.
The DLCI carried within the Frame Relay header is modified as it
traverses the network. When the packet arrives at its destination,
the DLCI has been set to the value that, from the standpoint of the
receiving station, corresponds to the sending station. For example,
in figure 1 below, if station A were to send a message to station B,
it would place DLCI 50 in the Frame Relay header. When station B
received this message, however, the DLCI would have been modified by
the network and would appear to B as DLCI 70.
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RFC 1294 Multiprotocol over Frame Relay January 1992
will be invalid. The address found in the frame header will,
however, be correct. Though it does violate the purity of layering,
Frame Relay may use the address in the header as the sender hardware
address. It should also be noted that the target hardware address,
in both ARP request and reply, will also be invalid. This should not
cause problems since ARP does not rely on these fields and in fact,
an implementation may zero fill or ignore the target hardware address
field entirely.
As an example of how this address replacement scheme may work, refer
to figure 1. If station A (protocol address pA) wished to resolve
the address of station B (protocol address pB), it would format an
ARP request with the following values:
ARP request from A
ar$op 1 (request)
ar$sha unknown
ar$spa pA
ar$tha undefined
ar$tpa pB
Because station A will not have a source address associated with it,
the source hardware address field is not valid. Therefore, when the
ARP packet is received, it must extract the correct address from the
Frame Relay header and place it in the source hardware address field.
This way, the ARP request from A will become:
ARP request from A as modified by B
ar$op 1 (request)
ar$sha 0x1061 (DLCI 70) from Frame Relay header
ar$spa pA
ar$tha undefined
ar$tpa pB
Station B's ARP will then be able to store station A's protocol
address and Q.922 address association correctly. Next, station B
will form a reply message. Many implementations simply place the
source addresses from the ARP request into the target addresses and
then fills in the source addresses with its addresses. In this case,
the ARP response would be:
ARP response from B
ar$op 2 (response)
ar$sha unknown
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA
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RFC 1294 Multiprotocol over Frame Relay January 1992
Again, the source hardware address is unknown and when the request is
received, station A will extract the address from the Frame Relay
header and place it in the source hardware address field. Therefore,
the response will become:
ARP response from B as modified by A
ar$op 2 (response)
ar$sha 0x0C21 (DLCI 50)
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA
Station A will now correctly recognize station B having protocol
address pB associated with Q.922 address 0x0C21 (DLCI 50).
Reverse ARP (RARP) [8] will work in exactly the same way. Still
using figure 1, if we assume station C is an address server, the
following RARP exchanges will occur:
RARP request from A RARP request as modified by C
ar$op 3 (RARP request) ar$op 3 (RARP request)
ar$sha unknown ar$sha 0x1401 (DLCI 80)
ar$spa undefined ar$spa undefined
ar$tha 0x0CC1 (DLCI 60) ar$tha 0x0CC1 (DLCI 60)
ar$tpa pC ar$tpa pC
Station C will then look up the protocol address corresponding to
Q.922 address 0x1401 (DLCI 80) and send the RARP response.
RARP response from C RARP response as modified by A
ar$op 4 (RARP response) ar$op 4 (RARP response)
ar$sha unknown ar$sha 0x0CC1 (DLCI 60)
ar$spa pC ar$spa pC
ar$tha 0x1401 (DLCI 80) ar$tha 0x1401 (DLCI 80)
ar$tpa pA ar$tpa pA
This means that the Frame Relay interface must only intervene in the
processing of incoming packets.
In the absence of suitable multicast, ARP may still be implemented.
To do this, the end station simply sends a copy of the ARP request
through each relevant DLC, thereby simulating a broadcast.
The use of multicast addresses in a Frame Relay environment is
presently under study by Frame Relay providers. At such time that
the issues surrounding multicasting are resolved, multicast
addressing may become useful in sending ARP requests and other
"broadcast" messages.
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RFC 1294 Multiprotocol over Frame Relay January 1992
Because of the inefficiencies of broadcasting in a Frame Relay
environment, a new address resolution variation was developed. It is
called Inverse ARP [11] and describes a method for resolving a
protocol address when the hardware address is already known. In
Frame Relay's case, the known hardware address is the DLCI. Using
Inverse ARP for Frame Relay follows the same pattern as ARP and RARP
use. That is the source hardware address is inserted at the
receiving station.
In our example, station A may use Inverse ARP to discover the
protocol address of the station associated with its DLCI 50. The
Inverse ARP request would be as follows:
InARP Request from A (DLCI 50)
ar$op 8 (InARP request)
ar$sha unknown
ar$spa pA
ar$tha 0x0C21 (DLCI 50)
ar$tpa unknown
When Station B receives this packet, it will modify the source
hardware address with the Q.922 address from the Frame Relay header.
This way, the InARP request from A will become:
ar$op 8 (InARP request)
ar$sha 0x1061
ar$spa pA
ar$tha 0x0C21
ar$tpa unknown.
Station B will format an Inverse ARP response and send it to station
A as it would for any ARP message.
11. IP over Frame Relay
Internet Protocol [9] (IP) datagrams sent over a Frame Relay network
conform to the encapsulation described previously. Within this
context, IP could be encapsulated in two different ways.
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RFC 1294 Multiprotocol over Frame Relay January 1992
will follow immediately. The frame would be as follows:
+----------------------+----------------------+
| Q.922 Address |
+----------------------+----------------------+
| Control (0x03) | NLPID - 0x81 (CLNP) |
+---------------------------------------------+
| CLNP packet |
| . |
| . |
+---------------------------------------------+
13. Bridging in a Frame Relay network
A Frame Relay interface acting as a bridge must be able to flood,
forward, and filter packets.
Flooding is performed by sending the packet to all possible
destinations. In the Frame Relay environment this means sending the
packet through each relevant DLC.
To forward a packet, a bridge must be able to associate a destination
MAC address with a DLC. It is unreasonable and perhaps impossible to
require bridges to statically configure an association of every
possible destination MAC address with a DLC. Therefore, Frame Relay
bridges must provide enough information to allow a Frame Relay
interface to dynamically learn about foreign destinations beyond the
set of Frame Relay stations.
To accomplish dynamic learning, a bridged packet shall conform to the
encapsulation described within section 7. In this way, the receiving
Frame Relay interface will know to look into the bridged packet and
learn the association between foreign destination and Frame Relay
station.
14. For Future Study
It may be desirable for the two ends of a connection to have the
capability to negotiate end-to-end configuration and service
parameters. The actual protocol and parameters to be negotiated will
be a topic of future RFCs.
15. Backward Compatibility
This section is included in this RFC for completeness only. It is
not intended to suggest additional requirements.
Some existing Frame Relay stations use the NLPID value of 0xCE to
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RFC 1294 Multiprotocol over Frame Relay January 1992
indicate an escape to Ethernet Packet Types as defined in the latest
version of the Assigned Numbers (RFC-1060) [7]. In this case, the
frame will have the following format:
+-----------------------------+
| Q.922 Address |
+-- --+
| |
+-----------------------------+
| Control (UI = 0x03) |
+-----------------------------+
| Optional Pad(s) (0x00) |
+-----------------------------+
| NLPID (0xCE) |
+-----------------------------+
| Ethertype |
+- -+
| |
+-----------------------------+
| . |
| . |
| Data |
| . |
| . |
+-----------------------------+
| Frame Check Sequence |
+-- . --+
| (two octets) |
+-----------------------------+
The Ethertype field is a 16-bit value used to identify a protocol
type for the following PDU.
In order to be fully interoperable with stations that use this
encoding, Frame Relay stations may recognize the NLPID value of 0xCE
and interpret the following two byte Ethertype. It is never
necessary to generate this encapsulation format only to properly
interpret it's meaning.
For example, IP encapsulated with this NLPID value will have the
following format:
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